In vehicle windscreen wiper motors, it is known to connect to each supply brush of the motor a choke and a capacitor so as to suppress radio frequency interference caused during use of the windscreen wipers. In use, the choke suppresses frequencies in the FM broadcast frequency band and higher frequencies, while the capacitor suppresses the AM broadcast band and lower frequencies.
The choke is commonly formed of a length of copper wire wound into a coil around a rod-shaped core of magnetically conductive material, such as a ferrite. The number of turns of the coil is generally chosen as a compromise between the greatest number of turns which can be carried by the core, the current carrying capacity of the wire and the space available in the motor housing. In order to ensure that the core does not work itself loose from the coil, it is common to apply a coat of a suitable varnish between the coil and core so as to bond the coil to the core. Alternatively, the core can be secured in place by bending the ends of the coil around the ends of the core, although this is generally more time consuming as an additional manufacturing step is required after fitting the coil onto the core.
For a windscreen wiper motor adapted to provide two or more wiping speeds, the motor includes two or more supply brushes and, consequently, an interference suppression choke and a capacitor connected to each supply brush.
It is possible to use a single capacitor for all of the supply brushes, by coupling the capacitor directly to the supply line which connects the motor to a power supply, in such a manner that the brush selection switch used to switch from one supply brush to another is located between the capacitor and the supply brushes.
On the other hand, in order for the chokes to provide adequate noise suppression, it is necessary for them to be positioned close to the source of noise, that is close to their respective supply brush. As it is not normally possible to place the brush selection switch close to the supply brushes, due in part to space restrictions within the motor housing and to the need to have the switch close to the driver, it is not possible to reduce the number of chokes.